1. Field of the Invention
The present invention relates to a system, method, and program product for estimating the state of a secondary battery used in various electronic devices, electrically powered devices, etc.
2. Description of the Related Art
In recent years, due to global warming and concerns about the depletion of oil resources, a transition towards a low-carbon society has been demanded. As one of the activities towards such a transition, the importance of secondary batteries is increasing in industrial areas, such as electric power transactions using secondary batteries in electric power grids, peak shifts using secondary batteries in factories, and the change of power systems from internal combustion engines using energy from oil to electric motors using energy from electric power.
However, there is a problem in that the repeated charging and discharging of secondary batteries causes gradual decreases in the charging rate. A reduction in the performance of a secondary battery leads to a reduction in the range and other travel functions of a motor vehicle in which the secondary battery is used as a driving source, and causes safety problems. Therefore, various technologies for estimating the state of a secondary battery have been proposed in the related art.
Japanese Patent Application Publication No. 9-215207 discloses a technology in a system for monitoring a battery having discharging/charging cycles, for providing predictive information relating to the instant at which a predetermined threshold of the battery discharge voltage will be reached using a neural network.
Japanese Patent Application Publication No. 11-32442 discloses a technology in which, in order to allow the charge and discharge voltage and current of a load such as a motor and a storage battery to be digitally processed, the voltage and the current are converted into digital signals by different ND converters; the current and the voltage are further converted into complex spectra by a voltage frequency converter and a current frequency converter; impedance is calculated by an impedance calculation unit from the obtained complex spectra of the voltage and the current when the storage battery is in use; a radius, which is an impedance characteristic, is determined from the storage battery that is in operation; the determined radius is compared with a radius determined in advance and stored in a battery remaining capacity calculation unit; and the remaining capacity of the storage battery is estimated from the relationship between the radii.
Japanese Patent Application Publication No. 2002-319438 discloses a technology in which, in order to achieve an accurate and reproducible estimation of the state of charge of a battery to successfully operate a hybrid power train of a vehicle incorporating a battery pack: a state vector that describes the state of the battery is generated; a response to the state vector is estimated, a battery response is measured; and the state vector is modified on the basis of the difference between the estimated response and the measured response to determine the state of the battery.
Japanese Patent Application Publication No. 2011-38857 discloses a technology relating to a capacity maintenance ratio determination device capable of accurately determining a capacity maintenance ratio of a battery within a short period of time without completely charging/discharging the battery. The capacity maintenance ratio determination device includes an impedance measurement unit and a capacity estimation unit, an alternating signal is supplied to the battery from a signal generation unit, and the impedance measurement unit calculates the frequency characteristics of the AC impedance on the basis of a response signal from the battery in response to the alternating signal. A feature frequency is determined from the calculated frequency characteristics. The capacity estimation unit includes a memory and a determination unit. The memory stores relationships among the temperature of the battery, the feature frequency, and the capacity maintenance ratio. The determination unit determines the capacity maintenance ratio of the battery, based upon the temperature of the battery that is detected by a temperature detection unit, the determined feature frequency, and the relationships stored in the memory.
The related art publications described above disclose technologies for estimating the performance of a battery on the basis of an impedance characteristic of the battery in operation, the frequency characteristics of the AC impedance measured on the basis of a response signal from the battery, the temperature of the battery, etc. However, these technologies are not battery performance estimation technologies that take the internal battery state into account nor do they take the battery use history into account, thus causing an inaccurate prediction of deterioration of the battery.
One parameter of state known as the internal battery state is an AC impedance curve plot. This is a plot of values representing two components of impedance, including the original resistance, capacitance, and frequency. The resistive component and capacitive component of impedance are measured at various frequencies and are plotted on the horizontal axis and vertical axis, respectively. Thus, a semicircle is obtained, where the diameter of the semicircle represents the original resistance. The use of the above measured values allows more accurate prediction of the deterioration of a battery. However, there exists a problem in that a measurement device for measuring an AC impedance curve plot is expensive. For the foregoing reasons, it is difficult to readily measure the internal battery state when a secondary battery is in use. The internal battery state can also be measured at limited timings such as during maintenance or during recycling of the battery.